This invention relates to a copper thick film conductor paste and electrical conductor elements made therefrom, and also relates to a method of making the paste and to a method of making an electrical conductor element from the paste.
In the manufacture of electrical circuits, it has long been desired to provide a screen-printable copper conductor which could be used instead of more expensive conventional materials such as gold.
It has been desired to provide a screen-printable copper conductor that fires at conventional furnace temperatures, costs less than precious-metal conductors, and adheres strongly to substrates made of a variety of different materials which may be classified under four group types: single crystalline materials such as saphires, diamonds, niobates, tantalates, titanates, rutiles; poly crystalline materials such as ceramics which include for example, porcelains, steatites, aluminas, fosterites, ferrites; the amorphous materials such as silicate glasses; and materials known as "cermets", such as chromium - chromium oxide, which are a combination of ceramic and metal.
One of the problems with prior art copper thick film conductors is that they are not compatible with resistors.
It is known in the prior art to provide a conductive layer of copper on a substrate by spraying, dipping or screening a suspension of copper or copper oxide and powdered glass particles in a volatile organic suspension medium, drying said deposited film, and firing the film and substrate to remove any organic residual materials, to soften the glass powder and cause the glass portion of the suspension to establish a bond between the conductive metal film and the substrate, and to sinter the metal particles.
The prior art teaches that in the case where copper oxide is used, a layer of copper oxide is first established and a subsequent reduction firing is required to convert the oxide to free copper, which is a conductor. See U.S. Pat. No. 2,993,815, issued July 25, 1961 to Treptow which describes a copper process, and further describes a glass composition which resists chemical change under reducing environments in the firing oven.
Prior copper thick film conductor compositions have not performed well. For example, typically they withstand only 2-3 pounds peel on 80 mil square test pad, and have low conductivity of about 10 milliohms/square/mil sheet resistivity.